1. Field of the Invention
The present invention relates to multiplexing apparatus and methods and more particularly to passive multiplexing whereby a multiplicity of signals may be multiplexed without employing electrically active devices at the information generating terminals.
2. Description of the Prior Art
Underwater target locating systems may utilize active acoustic devices which comprise a radiation source and a receiver to detect reflected sound waves or passive devices which detect sound waves emanating from distance sources. The information obtained with active and passive systems is the same; mainly they determine a relative position of a multiplicity of dispersed discrete targets in a quiescent water ambient. Passive underwater locating systems are generally preferred for military applications. Since target position cannot be determined passively with a single acoustic receiver (hydrophone), a passive system requires a multiplicity of hydrophones, the minimum number being 3, one each positioned at the vertices of an equal angular triangle. These systems determine the relative position of a target by frequency filtering the target signal received at each sensor from the background noise, determining the relative signal phases at the sensors, and processing this phase information to obtain the relative range and angle location. Large numbers of hydrophones, 100 to 1000, are typically utilized in one or two dimensional arrays to provide sufficient signal sensitivity in the presence of ambient noise and to provide desired angular resolution over a broad acoustic frequency band.
In the prior art the hydrophones comprised piezolectric crystals to transform acoustic signals into electrical signals by converting pressure variations at the crystal into corresponding voltage variations across electrodes positioned on opposite sides of the crystal. These transducers typically supply very small voltages at very high output impedence levels. Generally each transducer is coupled to an amplifier in close proximity thereto which amplifies the signal and transforms the high output impedence of the transducer to a low output impedence which is coupled to the input terminals of a transmission line that, for example, may be a coaxial cable.
Acoustically passive hydrophone arrays are generally utilized for long distance covert sensing. Since the attenuation of acoustic waves in water increases rapidly with increasing frequency, the operating frequency band of an acoustically passive sensing array is generally limited to be within the range between 10 Hz to 10 KHz. The bandwidth capability of coaxial cable is significantly greater than 10 KHz, a characteristic that may be utilized to reduce the number of cables employed in the array. Coaxial cables exhibit signal attenuations of from 20 to 200 dB/Km for an operating bandwidth of 100 MHz and from 2 to 20 dB/Km for a bandwidth of 10 MHz. The ten MHz bandwidth would be preferred if the signals that propagate along the coaxial cable are of limited strength and the length of the cable exceeds 1 Km. With this cable bandwidth approximately one thousand 10 KHz bandwidth signals can be transmitted over the same cable. Electrical power for the transimpedence amplifiers and for multiplexing several acoustic sensors onto one transmission line must be transmitted through a sensor cable structure which must also contain the coaxial cable therewithin for returning the multiplexed signals to a common processing point. If the sensors are dispersed, transimpedence amplifiers must be located adjacent to each to establish sufficient signal to noise ratio for sensitive signal detection at the processing point. Coupling apparatus for injecting the amplified electrical signal from each transducer onto the coaxial cable without disturbing the signals from all the other transducers must also be provided. This coupling may be achieved with the utilization of the electrical signal generated at the output terminal of a transducer in response to a received acoustic signal to modulate a specific carrier signal at a frequency within the 10 MHz bandwidth and to couple this modulated carrier signal via a T junction and a tuned amplifier onto the coaxial cable. It is therefore apparent that acoustic sensor arrays are complicated systems comprising acoustical signal to electrical signal transducers, electronics for impedence conversion, electronics for multiplexing, cables for carrying power lines, coaxial cables for the propagation of electrical signals, and mechanical members for maintaining the various elements within a specified package.
The application of fiber optic technology has been investigated in an effort to simplify underwater target locating systems. One such application utilizes color as a means of multiplexing without requiring electrical power at the sensor location. In this system the emissions of a multiplicity of lasers, each operating at a distinct optical wavelength, are combined by a color (wavelength) multiplexer onto a common fiber. Filters (color demultiplexers) positioned at acousto-optic transducers, which are utilized as the hydrophones, couple each color to its assigned sensor. Modulated optical signals from the sensors are then coupled to the fiber (remultiplexed) whereon they propagate, jointly to a common receiving location whereat the colors are separated (demultiplexed) to provide individual hydrophone signals. This type of optical multiplexer employs one laser (light source), one detector, and at least one multiplexer unit for each sensor and is limited to low multiplexing ratios. Color multiplexing, moreover, does not effectively utilize the available channel capacity. These disadvantages cause the color multiplexer to be complex and expensive compared with alternative multiplexing systems. The present invention is directed to an electrically passive optical multiplexing system which is simple and inexpensive.